The invention generally relates to wireless networks. More specifically, the invention relates to multiple-input-multiple-output (MIMO) communication in wireless networks.
A MIMO wireless communication system comprises multiple antennas at the transmitter and the receiver. The use of multiple antennas increases the transmission capacity of the system as compared to a single-input-single-output (SISO) system. Thereby, increasing the number of users as well as the amount of data that can be sent by each user.
The antennas work best in a multi-path environment such that the streams, in which the data is transmitted and received, are uncorrelated. Closely spaced antennas in user-equipment lead to increased correlation between the data streams that are communicated by them. Due to the severe space constraints in user-equipment, it is not possible to accommodate a large number of sufficiently spaced antennas that are uncorrelated with each other.
This problem of inter-antenna correlations can be handled by various conventional methods. The paper titled ‘Multi-user Space-Time Block Coded MIMO System with Unitary Downlink Precoding’ by Runhua Chen, Jeffrey G. Andrews, and Robert W. Heath, Jr, describes one such method. The paper was published in Proceedings of IEEE International Conference on Communications, volume 5, pages. 2689-2693, in June 2004. This method suggests the use of a downlink precoder, to eliminate co-channel interference. The precoder eliminates co-channel interference at the base station itself so that single-antenna user-equipments can be used. However, the method operates well only when the transmitter at the base station has the complete channel state information.
Another conventional method tries to resolve the problem of inter-antenna correlation by assigning the different user-equipments present in the vicinity to a group. The data of the user-equipments is selectively beam-formed to the group by the base station. According to one such method, beamforming weight vectors are calculated. These vectors are multiplied to the signal so that the interference between the streams, of data can be avoided. However, the method operates satisfactorily only when channel state information is available at the base station. Moreover, as the streams transmitted by different antennas are correlated, beamforming cannot achieve the full capacity offered by MIMO communications.
Another conventional method is disclosed in the U.S. patent application Ser. No. 20040114618 titled ‘Virtual MIMO Communication System’ assigned to ‘Nortel Networks Limited’. The method uses single antenna user-equipments for achieving MIMO communication. According to the method, the user-equipments are assigned to a group by the base stations. The user-equipments inter-transmit their data. The data of two or more user-equipments is cross-coded and combined into a single block. This block is then sent to the base station. Thereafter, the base station decodes the block to get the data of each user-equipment. However, the method is useful only when the data of two or more user-equipments is to be sent to the base station. Moreover, it can be used to obtain only as many antennas for communication as the number of antennas available with the users in the vicinity that are communicating during that period.
In view of the above discussion, there is a need for a MIMO communication method with low inter-antenna correlation that can function with the data of one or more user-equipments. Further, there is a need for a MIMO communication method in which the number of antennas transmitting or receiving is greater than the number of users in the same vicinity, communicating at a certain period, in one time, frequency, code or any other similar slot.